1. Field of the Invention
The present invention relates to an electromagnetic wave absorber used for an anechoic chamber and so on.
2. Description of the Related Art
An anechoic chamber has been used as a facility for testing electromagnetic compatibility (EMC), such as measuring electromagnetic wave noise generated by an electronic apparatus with accuracy. The anechoic chamber has four walls and a ceiling on entire surfaces of which electromagnetic wave absorbers are placed to prevent electromagnetic waves from reflecting off the surfaces of the walls and the ceiling. In this chamber a turntable is provided, and measurement of radiation noise generated by an electronic apparatus placed on the turntable is performed, for example.
An electromagnetic wave absorber that is widely used for an anechoic chamber is made up of: an electromagnetic wave reflector of metal; a ferrite tile, that is, an electromagnetic wave absorber made of plate-shaped (tile-shaped) sintered ferrite; and a wedge-shaped or pyramid-shaped dielectric loss material. These components are stacked in this order.
There are anechoic chambers of different sizes, such as a 10 meter-test-range chamber and a 3 meter-test-range chamber, since the distance between a measuring instrument and an electronic apparatus varies depending on the size and so on of the electronic apparatus whose radiation noise is to be measured.
The regulation imposed on the radiation noise of electronic apparatuses strongly demands that emission of strong electromagnetic waves to the outside should be avoided. Therefore, to perform measurement of radiation noise with high reliability, a high-quality anechoic chamber that exhibits high measurement accuracy, stability and repeatability is required.
When an anechoic chamber facility is built, it is desired that each wave absorber is as small as possible in size, considering the limitations imposed on the construction site and costs. When radiation noise is measured, however, the region required for measurement depends on the area of the metal floor surface allocated and the size of the measuring antenna and so on. It is thus required to make the thickness of each of wave absorbers placed on the entire inner walls of the chamber as small as possible, while maintaining the region required for measurement in the chamber.
As thus described, thin electromagnetic wave absorbers that exhibit an excellent electromagnetic wave absorbing characteristic are desired for anechoic chambers.
FIG. 13 and FIG. 14 illustrate examples of external appearances of electromagnetic wave absorbers that have been used for the 10 meter-test-range chamber and the 3 meter-test-range chamber. Each of the absorbers shown is made up of an electromagnetic wave reflector 101 of metal, a ferrite tile 102 and a dielectric loss material 103 that are stacked in this order. The dielectric loss material 103 of the wave absorber of FIG. 13 is wedge-shaped. The dielectric loss material 103 of the wave absorber of FIG. 14 is pyramid-shaped. The thickness of a practically available wave absorber is about 100 to 200 cm for the 10 meter-test-range chamber and about 40 to 60 cm for the 3 meter-test-range chamber.
The electromagnetic wave absorber made of the combination of ferrite and dielectric loss material, such as the ones shown in FIG. 13 and FIG. 14, requires the dielectric loss material having a some degree of thickness, to obtain well-balanced wave absorbing characteristics in both a VHF band of 30 to 300 MHz and a UHF band of 300 MHz and greater. Therefore, the minimum thickness of the dielectric loss material of the wave absorber is about 100 cm for the 10 meter-test-range chamber, and about 30 cm for the 3 meter-test-range chamber. As thus described, it is difficult in prior art to implement thin wave absorbers that exhibit excellent electromagnetic wave absorbing characteristics.
A recently-known electromagnetic wave absorber utilizes a composite magnetic loss material, in place of the dielectric loss material, which is made of a resin such as polypropylene mixed with ferrite powder and formed into a hollow pyramid. However, the electromagnetic wave absorbing characteristic of this wave absorber is reduced in the vicinity of 30 MHz, that is, the minimum frequency of radiation noise measurement. It is therefore difficult to apply this absorber to an anechoic chamber that requires high-level characteristics. To solve this problem, the thickness of the absorber may be increased. However, if the thickness of the absorber is increased, the size of the chamber is required to be increased to obtain a greater effective space in the chamber, which will impose a limitation on the building site and increase building costs. In addition, the composite magnetic loss material simply made of a resin mixed with ferrite powder requires an increase in proportion of the ferrite powder or an increase in thickness of the magnetic loss substance in order to increase the magnetic loss. Manufacturing costs are thereby increased.
It is an object of the invention to provide an electromagnetic wave absorber having a reduced thickness and an excellent electromagnetic wave absorbing characteristic.
An electromagnetic wave absorber of the invention comprises: an electromagnetic wave reflector; a first electromagnetic wave absorber section made of plate-shaped sintered ferrite and having surfaces one of which is adjacent to the wave reflector; and a second electromagnetic wave absorber section adjacent to the other surface of the first wave absorber section. The second wave absorber section includes: a structure including a plurality of plate-shaped elements each of which is made of a magnetic loss material, the elements having surfaces located with specific spaces each of which is created between adjacent ones of the elements, the surfaces being located in a direction intersecting the other surface of the first wave absorber section; and a dielectric loss material located in at least part of the spaces each of which is created between adjacent ones of the elements of the structure.
According to the electromagnetic wave absorber of the invention, an excellent electromagnetic wave absorbing characteristic is achieved by the first and second wave absorbing sections. In addition, it is possible to reduce the thickness of the second wave absorbing section and to thereby reduce the entire thickness of the wave absorber.
According to the electromagnetic wave absorber of the invention, the surfaces of the elements of the structure may be orthogonal to the other surface of the first wave absorber section.
According to the electromagnetic wave absorber of the invention, the second wave absorbing section may be entirely plate-shaped.
According to the electromagnetic wave absorber of the invention, each of the elements of the structure may have a thickness in a range of 0.5 to 2.5 mm inclusive, and the spaces each of which is created between adjacent ones of the elements may fall within a range of 10 to 50 mm inclusive.
According to the electromagnetic wave absorber of the invention, each of the elements of the structure may have a permittivity of 10 or smaller.
According to the electromagnetic wave absorber of the invention, the elements of the structure may contain ferrite. In this case, the ferrite that the elements contain may be Nixe2x80x94Zn-type ferrite or Mnxe2x80x94Mgxe2x80x94Zn-type ferrite, and an amount of the ferrite that the elements contain may fall within a range of 70 to 85 weight % inclusive.
According to the electromagnetic wave absorber of the invention, the dielectric loss material may contain conductive particles.
According to the electromagnetic wave absorber of the invention, each of the spaces created between adjacent ones of the elements may be filled with the dielectric loss material. In this case, the dielectric loss material may contain conductive particles, and an amount of the conductive particles that the dielectric loss material contains may fall within a range of 1 to 11 grams per liter inclusive.
According to the electromagnetic wave absorber of the invention, the surfaces of the elements of the structure may be covered with the dielectric loss material. In this case, the dielectric loss material may contain conductive particles, and an amount of the conductive particles in an entire volume of the second wave absorbing section including an interior space thereof may fall within a range of 1 to 11 grams per liter inclusive.
According to the electromagnetic wave absorber of the invention, the following configuration is possible. Each of the elements of the structure is made of a base that is made of a hydrous inorganic compound or a resin and contains ferrite powder. The surfaces of the elements of the structure are orthogonal to the other surface of the first wave absorber section. The elements are arranged in one direction, or arranged to form any of a grid, a honeycomb and a corrugated shape. The dielectric loss material is made of a base that is made of a foam, a hydrous inorganic compound or a resin and contains carbon or graphite.
Other and further objects, features and advantages of the invention will appear more fully from the following description.